Given the fuel economy goals of future passenger vehicles, friction and weight reduction are of high importance to automotive engineers. The drivetrain of an automobile consists of multiple arrays of gears and bearings to transfer the rotary motion of the internal combustion engine to the rotary motion of the wheels. The efficiency of the drivetrain has evolved very rapidly in recent years due in large part to improved designs of transmissions, transfer cases and differential units. The design of rolling element bearings has played a vital role in this efficiency gain and will continue to evolve with the innovations of drivetrain technology.
Rolling element bearing assemblies are typically circular in shape, and generally comprise of rolling elements, normally contained by a cage, disposed between inner and outer raceways. Rolling elements take many forms, including spherical balls, cylindrical rollers, needle rollers, or various other configurations, such as cone-shaped tapered rollers or barrel-shaped spherical rollers. Cages are often used to contain the rolling elements and guide them throughout the rotating motion of the bearing, but are not a necessity in some configurations. The material of a cage can vary from steel to plastic, depending on the application, duty cycle, along with noise and weight requirements.
The type of bearing used for a particular application depends on multiple factors including the load, load direction, required stiffness, and speed. Angular contact ball bearings are known and are able to withstand combined radial and axial loads. One or two rows of balls are possible in a single bearing unit and various arrangements of multiple angular contact ball bearings are possible to address the needs of the application. Referring to FIG. 11, a cross-sectional view of a prior art angular contact ball bearing 200 is shown. Angular contact ball bearing 200 contains an inner ring 214, an outer ring 211, balls 212, and a ball cage 213. The inner ring 214 contains an inner ball raceway 219 with an axial shoulder 217; the outer ring 211 contains an outer ball raceway 218 with an axial shoulder 215. In addition to the inherent radial load capability of a ball bearing, the presence of the respective axial shoulders 217, 215 provides axial load capability in the directions shown in FIG. 11. In general, angular contact ball bearings provide a low-friction solution for drivetrain applications due to a rolling interface that is maintained between the balls 212 and respective inner and outer raceways 219, 218, including axial shoulders 217, 215, when subjected to radial or axial load conditions. However, the load capacity of angular contact ball bearings is relatively low compared to other bearing types.
Like angular contact ball bearings, tapered roller bearings are known and are also able to withstand combined radial and axial loads, but, for a given bearing envelope size, have a significantly higher load capacity than angular contact ball bearings. Referring to FIG. 10, a cross-sectional view of a prior art tapered roller bearing 100 is shown that rotates about a central axis 113. Tapered roller bearing 100 contains an inner ring 104, an outer ring 101, tapered rollers 102 and a tapered roller cage 103. The inner ring 104 contains an inner tapered roller raceway 108 with a small diameter end 112 and a large diameter end 111; a rib 106 is present at the small diameter end 112 and a rib 105 is present at the large diameter end 111. The outer ring 101 contains an outer tapered roller raceway 107 with a small diameter end 110 and a large diameter end 109. The design of tapered roller bearings is such that the inner raceway 108 and outer raceway 107 are angled with respect to the central axis 113 of the tapered roller bearing 100. For a given width of envelope space, the angled inner and outer raceways 108, 107 increase the amount of line contact with the tapered rollers 102 which increases the load capacity of the tapered roller bearing 100. The rib 106 on the small diameter end 112 of the inner raceway 108 is present to retain the tapered rollers 102 on the inner raceway 108. The rib 105 on the large diameter end 111 of the inner raceway 108 serves as a thrust interface for the tapered rollers 102. However, the friction that results from this sliding interface exceeds that of the rolling interface between the balls and axial shoulders of an angular contact ball bearing.
For drivetrain applications, tapered roller bearings offer high load capacity while sacrificing efficiency. A bearing solution is required that maintains the load capacity of a tapered roller bearing while lowering the inherent friction.